Title of Invention

"STAGED COMBUSTION METHOD FOR A LIQUID-FUEL AND AN OXIDANT IN A FURANCE"

Abstract

A combustion method for a liquid fuel and an oxidant, in which at least one jet of liquid fuel in atomized form and at least one jet of oxidant are injected, the oxidant jet comprising a primary oxidant jet and a secondary oxidant jet, and the primary oxidant jet being injected close to the liquid fuel jet so as to cause a first incomplete combustion, the gases emanating from this first combustion still containing at least part of the fuel, whereas the secondary oxidant jet is injected at a distance l2 from the liquid fuel jet, this being greater than the distance between the liquid fuel jet and the primary oxidant jet, which is closest to the liquid fuel jet, so as to enter into combustion with that part of the fuel present in the gases emanating from the first combustion, Characterized in that the primary oxidant jet is divided into at least two primary jets:- at least a sheathing first jet of primary oxidant injected coaxially around the jet of liquid fuel in atomized form; and - at least a second jet of primary oxidant injected at a distance l1 from the liquid fuel jet, the distance l1 between the second primary oxidant jet and the liquid fuel jet being between 1.5Dg and b/2, DG representing the diameter of the circle with the same area as the area of the injector through which the sheathing first primary oxidant jet is injected.

Full Text

The present invention relates to staged combustion method for a liquid-fuel and an oxidant in a furnace. The performance of a method of combustion in an industrial furnace must meet two criteria: - limitation of the discharge of atmospheric pollutants (NOx, dust, etc.), the quantity of which must be below the limit set by legislation; - control of the temperature of the furnace walls and of the charge to be heated so as to meet simultaneously the constraints relating to quality of the product subjected to combustion and those relating to energy consumption. Changes in legislation regarding emissions of atmospheric- pollutants, especially nitrogen oxides, have resulted in substantial development in combustion technologies. A first method of combustion that limits the emission of NOx is oscillation combustion (EP-A1-0 524 880), which consists in making the fuel and/or oxidizer flow oscillate. By moving away from stoichiometry (1/1 ratio), the local temperature decreases, which results in a reduction in NOx. Another solution is staged combustion, in which the reactants are diluted in the main regions of the reaction: this makes it possible to move away from stoichiometric proportions and to prevent temperature peaks favourable to the formation of NOx (WO 02/081967). These examples of the prior art are solutions essentially adapted to the combustion of a gaseous fuel. In two-phase combustion using a liquid fuel and a gaseous oxidizer, the method of combustion comprises the additional steps of atomizing the liquid, followed by vaporization of the liquid drops so that the fuel that has become gaseous can react with the gaseous oxidizer. Various additional parameters will therefore influence the combustion, namely the type of atomizer used for example, but also, in the case of an injector with assisted atomization, the flow velocity of the atomizing gas, which will have an effect on the size of the drops and the quality of the atomization. Apart from the parameters directly associated with the atomization step, the method of combustion will be influenced by the mixing of the reactants, since mixing has an effect on the mode of combustion and on the formation of polluting emissions. Thus, the ratio of the vaporization length to the mixing length is an important parameter. The vaporization length is the distance needed to evaporate the liquid fuel drops - it depends on the size of the drops, their velocity and the nature of the liquid. The mixing length is the distance needed for the reactants, which are injected separately, to be mixed in a stoichiometric ratio. If the vaporization length is too long compared with the mixing length, the combustion is incomplete; this is what is called the "incomplete combustion" or "brush" mode. However, if the vaporization length is too short compared with the mixing length, the excessively rapid mixing results in high levels of nitrogen oxide; this is what is referred to as the "vaporization" mode. It is therefore preferable to be at the transition between these two modes (vaporization length/mixing length ratio close to 1). EP-B1-0 687 853 proposes a method of staged combustion of a liquid fuel. This method consists in injecting the liquid fuel in the form of a divergent spray making an angle at the outer periphery of less than 15° and in injecting the oxidizer in the form of two streams, namely a primary stream and a secondary stream, the primary stream having to have a low velocity, namely less than 61 m/s. This method has several drawbacks. Firstly, the use of such a low angle means the use of a high atomizing gas velocity, which creates high head losses and may impair flame stability. Secondly, owing to the low value of the spraying angle, the combustion mode is of the "vaporization" type and does not allow the reduction in NOx to be optimized. Lastly, this low value of the spraying angle does not allow the geometrical parameters of the flame to be continuously varied. However, it may be useful, depending on the charge, to modify the geometry of the flame so as to prevent in particular the local formation of a hot spot. It is an object of the present invention therefore to propose a method of staged combustion using a liquid fuel, making it possible to limit the formation of NOx while still keeping a stable flame. It is another object to propose a method of staged combustion using a liquid fuel making it possible to limit the formation of NOx and to have a high level of burner flexibility. For this purpose, the invention relates to a method of combustion of a liquid fuel and an oxidizer, in which at least one jet of the liquid fuel in atomized form and at least one oxidizer jet are injected, the oxidizer jet comprising a primary oxidizer jet and a secondary oxidizer jet, the primary oxidizer jet being injected near the liquid fuel jet so as to cause a first incomplete combustion, the gases emanating from this first combustion still comprising at least part of the fuel, whereas the second oxidizer jet is injected at a distance 12 from the liquid fuel jet that is greater than the distance between the liquid fuel jet and the primary oxidizer jet closest to the liquid fuel jet, so as to enter into combustion with the fuel part present in the gases emanating from the first combustion, in which the primary oxidizer jet is divided into at least two primary jets: - at least a shrouding first primary jet that is injected coaxially around the jet of liquid fuel in atomized form; and - at least a second primary oxidizing jet injected at a distance l1 from the atomized liquid fuel jet. Other features and advantages of the invention will become apparent from reading the description that follows. Embodiments and methods of implementation of the invention are given as non-limiting examples, illustrated by Figures 1 and 2 which are schematic views of a device for implementing the method according to the invention. The invention therefore relates to a method of combustion of a liquid fuel and an oxidizer, in which at least one jet of the liquid fuel in atomized form and at least one oxidizer jet are injected, the oxidizer jet comprising a primary oxidizer jet and a secondary oxidizer jet, the primary oxidizer jet being injected near the liquid fuel jet so as to cause a first incomplete combustion, the gases emanating from this first combustion still comprising at least part of the fuel, whereas the second oxidizer jet is injected at a distance 12 from the liquid fuel jet that is greater than the distance between the liquid fuel jet and the primary oxidizer jet closest to the liquid fuel jet, so as to enter into combustion with the fuel part present in the gases emanating from the first combustion, in which the primary oxidizer jet is divided into at least two primary jets: - at least a shrouding first primary jet that is injected coaxially around the jet of liquid fuel in atomized form; and - at least a second primary oxidizing jet injected at a distance l1 from the liquid fuel jet. One of the essential features of the method according to the invention is that it relates to a method of combustion of a liquid fuel, which is ejected from the lance of the burner in atomized form. This jet of liquid fuel in atomized form may be obtained by any method of atomization such as the pressurized ejection of the liquid fuel or mixing the fuel with an atomizing gas before or during its ejection. Thus, according to a preferred embodiment, the jet of liquid fuel in atomized form may be obtained by coaxial injection of an atomizing gas jet around a liquid fuel jet. The atomizing gas may be chosen from an oxidizing gas, such as air or oxygen, or an inert gas, such as nitrogen, or water vapour. According to this preferred embodiment, the mass flow rate of the atomizing gas jet is advantageously between 5 and 40%, even more preferably between 15 and 30%, of the mass flow rate of the liquid fuel jet. According to another essential feature of the invention, the primary oxidizing jet is divided into at least two jets, at least of which is a shrouding primary oxidizing jet. This shrouding primary oxidizing jet is injected coaxially around the jet of liquid fuel in atomized form. The second primary oxidizing jet is injected at a distance l1 from the atomized liquid fuel jet. Preferably, this distance l1 between the second primary oxidizing jet and the liquid fuel jet is between 1.5DG and l2/2, DG representing the diameter of the circle with the same area as the area of the injector through which the shrouding primary oxidizing jet is injected. As an example, the value of DG may be between 30 and 60 mm. The distance 12 between the secondary oxidizer jet and the fuel jet may be between 8D2 and 40D2, where D2 represents the diameter of the circle with the same area as the area of the injector through which the secondary oxidizer is injected. This diameter D2 may be between 10 and 60 mm. The diameter of the circle with the same area as the area of the injector through which the second primary oxidizer jet is injected, namely D1, may be between 15 and 70 mm. Preferably, the diameter D1 is greater than the diameter D2. According to a variant of the invention, the secondary oxidizer jet and the primary oxidizer jet located at a distance l1 from the liquid fuel jet consist of a plurality of jets. Thus, the primary oxidizer jet located at a distance l1 from the liquid fuel jet may consist of two identical jets located at the same distance l1 from the liquid fuel jet, the three jets lying substantially in the same plane, and the secondary oxidizer jet may consist of two identical jets located at the same distance 12 from the liquid fuel jet, the three jets lying substantially in the same plane; preferably, the five jets lie substantially in the same plane. The amount of secondary oxidizer generally represents at most 90%, preferably 10 to 90%, of the total amount of oxidizer injected. More preferably, the amount of secondary oxidizer is between 50 and 90%, or even between 60 and 80%, of the total amount of oxidizer injected, the primary oxidizer (which corresponds both to the shrouding oxidizer and the second primary oxidizer jet) representing an amount of between 10 and 50%, or even between 20 and 40%, of the total amount of oxidizer. Preferably, the mass flow rate of the shrouding first primary oxidizer jet is between 10 and 20% of the mass flow rate of the total oxidizer (primary oxidizer + secondary oxidizer) jet. The primary oxidizer and the secondary oxidizer may have the same composition; in particular, this has the advantage of having only a single source of oxidizer to be divided between the various primary or secondary oxidizer injection points. However, preferably, the oxygen concentration in the primary oxidizer is higher than the oxygen concentration in the secondary oxidizer. The composition of the oxidizer may vary, depending on the conditions or the results that are desired. In general, the oxidizer may consist of a gas mixture comprising: - from 5 to 100%, preferably 30 to 100%, oxygen by volume; - from 0 to 95%, preferably 0 to 90%, C02 by volume; - from 0 to 80%, preferably 0 to 70%, N2 by volume; and - from 0 to 90% Ar by volume. The mixture may also contain other constituents, especially water vapour and/or NOx and/or SOx. In general, the air provides 0 to 90% by volume of the total oxygen flow of the oxidizer, the balance being provided by oxygen-enriched air or substantially pure oxygen. Preferably, air represents 15 to 40% of the total oxidizer by volume, and particularly 15 to 40% of oxygen in the oxidizer by volume. According to an advantageous method of implementation, the injection velocities of the second primary oxidizer jet and the secondary oxidizer jet are less than or equal to 200 m/s, and when the method according to the invention is implemented for the combustion of a glass charge the injection velocities of the second primary oxidizer jet and the secondary oxidizing jet are preferably less than or equal to 100 m/s. Furthermore, it is preferable for the velocity of the secondary oxidizer jet to be greater than the velocity of the second primary oxidizer jet. Figure 1 is a partial schematic top view of an example of a combustion assembly for implementing the method according to the invention and Figure 2 is a corresponding sectional schematic view. The combustion assembly is placed in a refractory block 1 having three cylindrical bores 2, 3 and 4 into which three blocks 21, 31 and 41 have been respectively slipped. The block 21 comprises: - a duct (or injector) 211 that emerges at 22. This duct 211 receives the liquid fuel 212; - a duct (or injector) 221 that emerges at 22 and is placed concentrically around the duct 211 into which the liquid fuel 212 is injected. This duct 221 receives the atomizing gas 222; and - a duct (or injector) 231 that emerges at 22 and is placed concentrically around the duct 221 into which the atomizing gas 222 is injected. This duct has a diameter DG at 22. It receives the shrouding primary oxidizer 232. The preferably cylindrical block 31 is pierced by a duct (or injector) 32, the orifice of which emerges at 33 in the block. This duct (or injector) 32 has a diameter at 33 equal to D1 and the centre of this duct 32 is located at a distance 11. from the centre of the duct 211. The duct 32 receives the primary oxidizer (34) that differs from the shrouding primary oxidizer. The preferably cylindrical block 41 is pierced by a duct (or injector) 42, the orifice of which emerges at 43 in the block. This duct (or injector) 42 has a diameter at 43 equal to D2 and the centre of this duct 42 is located at a distance 12 from the centre of the duct 211. The duct 42 receives the secondary oxidizer 44. To operate this system, it is possible to use the same oxidizer source for the primary oxidizer 34 and 232 and the secondary oxidizer 44, the diameters of the corresponding ducts 32, 231 and 42 being chosen so as to set injection velocities that are different or identical depending on the type of combustion desired. According to an advantageous embodiment, the ends of the ducts for injecting oxidizers are set back in the refractory port. To implement the method according to the invention, it is possible for a liquid fuel to undergo combustion while the formation of NOx is still being limited. Furthermore, the method according to the invention has the advantage of allowing the stability of the flame and the thermal flexibility of the method to be controlled. This is because, depending on the nature of the charge and on the geometry of the furnace, it may be preferable to use a flame of small or large volume, or to control the heat transfer at certain points in the furnace, or to make the temperature of the crown uniform, etc. According to the invention, this flexibility is achieved by controlling the distribution of the total oxidizer flow between the secondary oxygen jet and the primary oxidizer jets, and preferably between the secondary oxygen jet and the second primary oxidizer jet which is different from the shrouding primary oxidizer jet. This control of the way in which the total oxidizer flow is distributed is also called staging. EXAMPLE A burner having the configuration shown in Figures 1 and 2 was used, this furthermore comprising: - a second primary oxidizer injector located at a distance l1 from the liquid fuel jet 211 and symmetrical with the first primary oxidizer injector 31 with respect to the fuel injector 211; and - a second secondary oxidizer injector located at a distance 12 from the liquid fuel injector 211 and symmetrical with the first secondary oxidizer injector 42with respect to the fuel injector 211. The five jets all lay in the same plane. The power of the burner was 2 MW. The burner was fitted into a furnace having a length of 6 m and a cross section of 1.5 m by 2 m. The 12/D2 ratio was 14.6, the l1/DG ratio was 2 and the l1/12 ratio was 0.26. The injected fuel was a heavy fuel oil having the following composition: 87.9 wt% C; 10.02 wt% H; 0.67 wt% 0; 0.39 wt% N; and 0.98 wt% S. Its dynamic viscosity was 39 mm2/s at 100°C, its density was 980 kg/m3 and its net calorific value was 9631 kcal/kg. The atomizing gas was either oxygen or air. By implementing the method according to the invention, it was possible to modify the geometry of the flame by controlling the distribution of the total oxidizer flow between the various primary and secondary injectors. Thus, by injecting 75% of the total oxidizer flow into the secondary oxidizer injectors, a large-volume flame was obtained. Likewise, by modifying this percentage through injection, it is possible to decrease the volume of the flame. Thus, depending on the nature of the charge and the place where the burner is fitted into the furnace, it is possible using the method of the invention to adjust the volume of the flame. Figure 3 shows the power transferred by the flame to the hearth of the furnace as a function of the distance from the burner for various proportions of the total oxidizer flow injected into the secondary injectors (50, 65 and 75% of the total oxidizer flow injected into the secondary injectors) . It may be seen that substantial staging (injection of a larger amount of oxidizer into the secondary injectors than into the primary injectors) reduces the power close to the burner and increases the transfer far from the injectors. Through the method according to the invention it is thus possible to modify the thermal transfer profile. This is one advantage of the method of the invention since this method can be adapted to various types of furnace geometry. In the case of the examples shown in Figure 3, the atomizing gas was oxygen. Figure 4 shows the crown temperature of the furnace along the longitudinal axis of the furnace as a function of the distance from the burner for various proportions of total oxidizer flow injected into the secondary injectors (50, 65 and 75% of the total oxidizer flow injected into the secondary injectors). It may be seen that substantial staging improves the crown temperature uniformity. In the case of the examples shown in Figure 4, the atomizing gas was air. Figure 5 shows the amount of NOx emitted as a function of the proportion of total oxidizer injected into the secondary injectors (staging) and for types of atomizing gas, namely oxygen and air. The curve relating to the use of oxygen as atomizing gas is identified by open diamonds and the curve relating to the use of air as atomizing gas is identified by black squares. It may be seen that with substantial staging, the NOx emission is 200 ppm if the atomizing gas is oxygen and 300 ppm if the atomizing gas is air. We claim 1. Method of combustion of a liquid fuel and an oxidizer, in which at least one jet of liquid fuel in atomized form and at least one oxidizer jet are injected, the oxidizer jet comprising a primary oxidizer jet and a secondary oxidizer jet, and the primary oxidizer jet being injected near the liquid fuel jet so as to cause a first incomplete combustion, the gases emanating from this first combustion still containing at least part of the fuel, whereas the secondary oxidizer jet is injected at a distance b from the liquid fuel jet, that is greater than the distance between the liquid fuel jet and the primary oxidizer jet, which is closest to the liquid fuel jet, so as to enter into combustion with the fuel part present in the gases emanating from the first combustion, characterized in that the primary oxidant jet is divided into at least two primary jets: - at least a shrouding first primary jet that is injected coaxially around the jet of liquid fuel in atomized form; and - at least a second jet of primary oxidizing jet injected at a distance l1 from the liquid fuel jet, And whereby the distance l1 between the second primary oxidizer jet and the liquid fuel jet being between l.5xDG and l2/2, DG representing the diameter of the circle with the same area as the area of the injector through which the shrouding first primary oxidizer jet is injected, b representing the distance b between the secondary oxidizer jet and the fuel jet. 2. Method as claimed in the preceding claim, wherein the jet of liquid fuel in atomized form is obtained by coaxial injection of a jet of an atomizing gas jet around a liquid fuel jet. 3. Method as claimed in claim 2, wherein the atomizing gas is chosen from an oxidizing gas, such as air or oxygen, or an inert gas, such as nitrogen, or water vapour. 4. Method as claimed in claim 2 or 3, wherein the mass flow rate of the atomizing gas jet is between 5 and 40% of the mass flow rate of the liquid fuel jet. 5. Method as claimed in one of the preceding claims, wherein the distance l2 between the secondary oxidizer jet and the fuel jet is between 8xD2 and 40xD2, where D2 represents the diameter of the circle with the same area as the area of the injector through which the secondary oxidizer is injected. 6. Method as claimed in one of the preceding claims, wherein the diameter of the circle with the same area as the area of the injector through which the secondary oxidizer is injected, namely D2, is between and 60 mm. 7. Method as claimed in one of the preceding claims, wherein the diameter of the circle with the same area as the area of the injector through which the second primary oxidizer jet is injected, namely D1, is between 15 and 70 mm. 8. Method as claimed in one of the preceding claims, wherein the total amount of secondary oxidizer is between 50 and 90% of the total amount of oxidizer injected. 9. Method as claimed in one of the preceding claims, wherein the injection velocities of the second primary oxidizer jet and the secondary oxidizer jet are less than or equal to 200 m/s. 10. Method as claimed in one of the preceding claims, wherein it is carried out for the combustion of a glass charge and in that the injection velocities of the second primary oxidizer jet and the secondary oxidizer jet are less than or equal to 100 m/s. 11. Method as claimed in one of the preceding claims, wherein the mass flow rate of the shrouding first primary oxidizer jet is between 10 and 20% of the mass flow rate of the total oxidizer (primary oxidizer + secondary oxidizer) jet.

Documents:

4626-DELNP-2005-Abstract-(08-06-2009).pdf

4626-delnp-2005-abstract.pdf

4626-DELNP-2005-Claims-(08-06-2009).pdf

4626-delnp-2005-claims.pdf

4626-DELNP-2005-Correspondence-Others-(08-06-2009).pdf

4626-DELNP-2005-Correspondence-Others-(16-03-2011).pdf

4626-DELNP-2005-Correspondence-Others-(17-06-2009).pdf

4626-DELNP-2005-Correspondence-Others-(18-06-2009).pdf

4626-delnp-2005-correspondence-others.pdf

4626-delnp-2005-desciption(complete).pdf

4626-DELNP-2005-Description (Complete)-(08-06-2009).pdf

4626-DELNP-2005-Drawings-(08-06-2009).pdf

4626-delnp-2005-drawings.pdf

4626-delnp-2005-form-1.pdf

4626-delnp-2005-form-18.pdf

4626-delnp-2005-form-2.pdf

4626-DELNP-2005-Form-27-(16-03-2011).pdf

4626-DELNP-2005-Form-3-(08-06-2009).pdf

4626-DELNP-2005-Form-3-(17-06-2009).pdf

4626-delnp-2005-form-3.pdf

4626-delnp-2005-form-5.pdf

4626-DELNP-2005-GPA-(08-06-2009).pdf

4626-delnp-2005-gpa.pdf

4626-DELNP-2005-Others-Documents-(17-06-2009).pdf

4626-delnp-2005-pct-210.pdf

4626-DELNP-2005-Petition-137-(08-06-2009).pdf

4626-DELNP-2005-Petition-138-(08-06-2009).pdf

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